The origin, nature and functions of the membranes separating the cytoplasm of intracellular parasites from that of their host cells will be investigated. Since these membranes are the site of all interactions between parasite and host cell an understanding of their physiology is of fundamental importance in the study of parasitism. Cytochemical methods at the electron microscope level will be applied to malaria parasites in erythrocytes, and also to Babesia. At the same time two other membrane systems in erythrocytic stages of mammalian malaria parasites will be studied: the concentric double- membraned structures which may have a mitochondrial function and the membrane-bounded clefts ramifying through the cytoplasm of the host erythrocyte, whose function has not even been guessed.